This invention relates to apparatus and methods for determining the attitude of a vehicle located in a gravitational field. In particular, but not exclusively, the invention relates to apparatus for determining the orientation of the Earth's gravitational vector with respect to an aircraft operating in the earth's gravitational field and thereby determining the pitch and bank angles of the aircraft.
There are at least two conventional techniques for establishing the attitude of a body in flight to the local horizontal plane. One such technique is Schuler tuning in which values of the spatial velocity of the body and its radius from the centre of the Earth are processed to determine the rate of rotation of the local horizontal plane as the body journeys over the globe. Since the Earth's rotation must be taken into account, the computation of the spatial velocity of the body requires a knowledge of direction of travel and position with respect to the earth's surface. Hence, this technique can only correctly be applied in conjunction with a navigation system.
A second such technique is to detect the Earth's gravitational vector by means of gravity seeking accelerometers. This task is complicated because dynamic acceleration can become confused with gravitational acceleration, and the latter should be isolated from the former. Conventional techniques attempt to separate the two by cutting out the accelerometer's gravity seeking function should the sensed acceleration exceed a certain value. To this end a vertical reference gyroscope, not Schuler tuned, is employed to retain the vertical reference until the extraneous acceleration is removed and the accelerometers can resume their gravity seeking function. This technique is prone to error if the inertial accelerations are within the accelerometer cutout threshold. As the vertical reference gyroscope is not Schuler tuned, and hence does not take account of angular movement about the centre of the Earth, this technique is limited to brief periods of manoeuvre interspersed with periods of steady flight to allow the correct vertical reference to be re-established, if sufficient accuracy is to be maintained. In addition, the vertical reference gyroscope has to be mounted in gimbals which are prone to gimbal lock resulting in toppling of the gyroscope and this imposes an operating limitation.